Cathode-ray tube having an electric cylinder lens for the dynamic correction of electrostatic deflection defocusing

ABSTRACT

A cathode-ray tube provided with an electric cylinder lens for the dynamic correction of electrostatic deflection defocusing. The lens is formed of two electrodes having slot-like apertures that are maintained at d-c voltages equal to the average voltage of the deflection plates. Interposed between the two electrodes is a flat, grid-like electrode having a negative voltage derived from the deflection voltage signal.

Uite States Patent 1191 1111 3,887,834

Himmelbauer June 3, 1975 CATHODE-RAY TUBE HAVING AN [56] References Cited ELECTRIC CYLINDER LENS FOR THE UNITED STATES PATENTS DYNAMIC CORRECTION 2,884,559 4/1959 Cooper et al. 315/14 ELECTROSTATIC DEFLECTION 2,995,676 8/1961 Schlesinger 315/30 DEFOCUSING 3,256,460 6/1966 Went 315/30 3.449, 24 6 1969 M l. 3 [75] inventor: Erich Eduard Himmelbauer, 6 ayo et a 15/31 TV Emdhoven Netherlands Primary Examiner-Maynard R. Wilbur {73] Assignee: US. Philips Corporation, New ss s ant Examin rN. MOSkOWitZ York, NY. Attorney, Agent, or FirmFrank R. Trifari; George B.

B k 221 Filed: 10a. 31, 1969 er [21] App]. No; 872,864 [57] ABSTRACT A cathode-ray tube provided with an electric cylinder [30] Foreign Application Priority Data lens for the dynamic correction of electrostatic deflec- N v 9 968 Netherlands 6816003 tion defocusing. The lens is formed of two electrodes 0 having slot-like apertures that are maintained at d-c voltages equal to the average voltage of the deflection lsz] m 315/31 315/ plates. Interposed between the two electrodes is a flat, Int Cl Holj 29/02 29/56 grid-like electrode having a negative voltage derived \[58] Field of Search 315/27 GD, 430; 313/82 fmm deflectlo voltage 1 Claim, 4 Drawing Figures SHEET INVENTQR. ERICH E.H IMMELBAUER AGE/VT CATHODE-RAY TUBE HAVING AN ELECTRIC CYLINDER LENS FOR THE DYNAMIC CORRECTION OF ELECTROSTATIC DEFLECTION DEFOCUSING The invention relates to a device employing a cathode-ray tube comprising an electron gun for producing an electron beam along the axis of the electron gun, a screen for receiving the beam, at least one pair of deflection plates for electrostatic deflection of the beam, which pair of plates is connected to a control device for controlling an electric cylinder lens for the dynamic correction of deflection defocusing. The invention furthermore relates to a cathode-ray tube for such a device.

It is known that a deflection defocusing occurs in a cathode-ray tube in the electrostatic deflection of an electron beam focused on the screen by means of deflection voltages between two deflection plates situated on either side of the beam in that the electrons, dependent upon their position in the beam cross-section, are deflected differently by the pair of deflection plates. The electrons moving nearer along the plate with the higher potential experience a larger accelerating voltage than the electrons moving at a larger distance from said plate along said plate, so that the first mentioned electrons have a shorter transit time between the deflection plates and hence are deflected to a smaller extent than the last-mentioned electrons. So as a result of the deflection a convergence occurs in the deflection plane, the intensity of which depends upon the deflection voltage. in the absence of a lens for the correction of said deflection defocusing, an undesirable line widening occurs on the screen. The deflection field has the property of an electric cylinder lens, which is to be understood to mean in this application an electron lens, the effect of which is analogous to that of an optical cylinder lens, in other words an electron lens which provides equal increase or decrease of the convergence in parallel planes. Since the deflection field has the property of a cylinder lens, the intensity of which still depends upon the deflection voltage, the use of an electric cylinder lens which is controlled with a correction voltage derived from the deflection voltage is suitable for an accurate correction of the deflection defocusing.

In known devices of the type mentioned in the first paragraph, converging cylinder lenses are used which comprise at least one electrode with an elongate aperture by means of which, in cooperation with adjacent electrodes, the cylinder lens field dependent upon the deflection voltage is produced for the correction of the deflection defocusing.

For example, such a device is known having an electron lens which comprises as the cylindrical lens element a pair of plates situated in one plane and separated from each other by a gap, said plates being situated opposite to a plate having a circular aperture and producing a cylindrical beam distortion in combination with said plate.

A device is also known employing an electron lens which comprises a plate having an elliptic aperture as the cylindrical lens element, which plate is situated opposite to a plate having a circular aperture and in combination therewith produces a cylindrical beam distortion.

The possibility has also been pointed out already of a device comprising a converging so-called Einzellens consisting of three plates having elongate slots for the correction of the focus in one direction.

In the above-mentioned known devices, an accurate correction of the deflection defocusing can be obtained, but they suffer from the drawback that they require a complicated control device for controlling the cylinder lens. The correction signals required for the cylinder lens are obtained from the deflection signals after amplification and suitable wave-shaping.

It is the object of the invention to provide a device in which this drawback is mitigated.

In a device of the type mentioned in the first paragraph, the cylinder lens according to the invention comprises two electrodes which are each provided with a slot-like aperture centered with respect to the axis of the electron gun, the long sides of said aperture being provided substantially at right angles to the direction of deflection of the said pair of deflection plates, a flat grid-like electrode being provided between said electrodes substantially at right angles to the axis of the electron gun.

For completeness sake it is stated that an electron lens which comprise two electrodes having slot-like apertures between which a grid-like electrode is placed and in which a negative voltage with respect to the outermost electrodes is set up at the grid-like electrode, is known per se as a diverging cylinder lens.

In the device according to the invention the direct voltage of each of the three electrodes of the cylinder lens can be chosen to be equal to the average voltage of the deflection plates and a correction signal which always has a negative polarity relative to said direct voltage can be derived from the deflection voltage signal and be applied to the grid-like electrode of the cylinder lens, so that the cylinder lens forms a divergent lens which produces a divergence of the electrons in the deflection plane, the strength of which divergence depends upon the strength of the deflection voltage. This cylinder lens can easily be proportioned so in practice that the characteristics, of the strength of the lens as a function of the correction voltage is such that the correction voltage required for the correction of the deflection defocusing is smaller than half the deflection voltage. When using deflection voltages in push-pull, in which one deflection plate has a positive signal relative to the average voltage of the deflection plates, and the other deflection plate at the same time has an equally large negative signal with respect to the average voltage of the deflection plates, a fraction of said negative signal is then sufficiently large to serve as a correction voltage signal. Moreover, the deviations from an ideal correction in a correction voltage which depends substantially linearly on said negative signal are so small that they can be neglected. Therefore a very simple control device without amplifiers and wave-shapers is possible.

The control device is preferably formed by two parallel rectifier elements which are each connected to one of the plates of the pair of deflection plates, and by a potentiometer which is arranged in series with the rectifier elements, and a tap of which is connected to the grid-like electrode of the cylinder lens.

The invention relates in particular to a cathode-ray tube for use in a device as described above. In such a cathode ray tube the cylinder lens may be arranged between the electron gun and the pair of deflection plates. Alternatively, the cylinder lens in such a cathode-ray tube may be arranged between the pair of deflection plates and the screen. Of course, the cathoderay tube may also comprise several pairs of deflection plates, all of which or some of which in combination with an associated cylinder lens of the type stated according to the invention.

Very particularly the invention relates to a cathoderay tube having at least two pairs of deflection plates situated at different distances from the electron gun for use in a device as described above, in which the lens elements of at least one cylinder lens associated with one of the pairs of deflection plates are situated between the said pairs of deflection plates. Said lens elements then form a screening of one pair of plates for the de flection field of the other pair of plates, and conversely. As a result of the presence of said lens elements, the length of said tube need be only slightly larger than that of any conventional type of tubes, in which already one electrode with a slot-like aperture is provided between two pairs of deflection plates for screening the deflection field.

In order that the invention may be readily carried into effect, one embodiment thereof will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:

FIG. I shows a construction ofa cathode-ray tube according to the invention which comprises two pairs of deflection plates and two cylinder lenses each associated with one of the pairs of deflection plates;

FIG. 2 is a perspective view of elements of the tube shown in FIG. 1, in which some proportions are exaggeratedly large, and in which diagrammatically control devices are shown which connect each pair of deflection plates to the associated cylinder lens;

FIGS. 3a and 3b show graphs for illustrating the operation of a cylinder lens in the tube shown in FIGS. 1 and 2.

Reference numeral 1 in FIG. 1 denotes the envelope of an evacuated cathode-ray tube. The tube comprises an electron gun 2 for producing an electron beam and a screen 3 for receiving the beam. The tube furthermore comprises a pair of plates (4,5) for the vertical deflection and a pair of plates (6,7) for the horizontal deflection. Between the electron gun 2 and the pair of plates (4,5) the cylinder lens is situated which comprises the electrodes 8, 9 and 10 for the dynamic correction of the deflection defocusing as a result of the vertical deflection. Between the pair of plates (4,5) and the pair of plates (6,7) the cylinder lens is situated which comprises the electrodes 11, 12 and 13 for the dynamic correction of the deflection defocusing as a result of the horizontal deflection. The supply lines to and the connection between the various electrodes are not shown in FIG. 1.

Of the elements shown in FIG. 1, FIG. 2 shows the electron gun 2, the two pairs of deflection plates (4,5)

and (6,7) and the two cylinder lenses (8,9,10) and which 24 is connected to the plate 4 and 25 is connected to the plate 5. The cylinder lens (11, 12, 13) which forms a screening for the deflection fields, is of the same type as the cylinder lens (8, 9, 10). The electrodes ll, 12 and 13 have apertures 18, 19 and 20, the long sides of which extend at right angles to the deflection of the pair of plates (6,7). The aperture 19 comprises a gauze 22 and the plate 12 is connected to the pair of plates (6,7) via a control device 44 which comprises a potentiometer 28 and the diodes 26 and 27. During operation, deflection signals are applied in push-pull to the pair of deflection plates (4,5) and (6,7) and a fraction of the negative signal is applied to the associated cylinder lenses via the associated control devices 33 and M, respectively.

The lenses have the following dimensions:

FIG. 3 shows as an example graphs for illustrating the properties of the cylinder lens (11,12,13) shown in FIGS. 1 and 2. In FIG. 3a the horizontal scan X of the electron beam on the screen with respect to the nondeflected electron beam is plotted in mms along the horizontal axis. Along the vertical axis of FIG. 3 is plotted a voltage V in volts. In FIG. 3b the horizontal scan X of the electron beam on the screen is plotted on the horizontal axis likewise in mms. Along the vertical axis of FIG. 3b the line width D of a vertical line produced on the screen by the electron beam is plotted in mms. The line 50 in FIG. 3a shows the relationship between half of the deflection voltage for horizontal deflection and the horizontal scan. The relation between the line width and the scan which is obtained when no correction is used is indicated by the line 51 in FIG. 3b. In the case of ideal correction of the deflection defocusing, the line width as a function of the scan would be represented by the line 52 in FIG. 3b. For that purpose a correction voltage of the electrode 12 of FIG. 2 would be required as is denoted by the line 53 in FIG. 3a. This correction voltage is smaller than half of the deflection voltage indicated by the line 50 in FIG. 3a and is approached by means of the device 44 in FIG. 2 by the correction voltage denoted by 54- in FIG. 3a. The line width varies along a line the ordinates of which deviate less than 10% from the line 52 of FIG. 3b.

What is claimed is:

l. A cathode-ray tube having means for the dynamic correction of defocussing caused by electrostatic deflection, comprising a hermetically sealed envelope, an electron gun within said envelope for producing an electron beam along the longitudinal axis of the electron gun, a target within said envelope for receiving said electron beam, at least one pair of deflection plates on opposite sides of a plane along which said axis lies within said envelope between said electron gun and said target for electrostatically deflecting said electron beam, means for applying deflection voltage signals to said deflection plates, and an electric cylinder lens surrounding said axis within said envelope for providing corrective deflection of said electron beam to compensate for defocussing of said electron beam resulting connection in series with said potentiometer, the cathode of each rectifier element being connected to each deflection plate for deriving from said deflection signals a negative voltage having a value as determined by departures of the amplitude of said deflection signals from the average value thereof, and the tap of said potentiometer being connected to said grid-shaped electrode thereby to provide dynamic correction of defocussing of said beam by said deflection signals. 

1. A cathode-ray tube having means for the dynamic correction of defocussing caused by electrostatic deflection, comprising a hermetically sealed envelope, an electron gun within said envelope for producing an electron beam along the longitudinal axis of the electron gun, a target within said envelope for receiving said electron beam, at least one pair of deflection plates on opposite sides of a plane along which said axis lies within said envelope between said electron gun and said target for electrostatically deflecting said electron beam, means for applying deflection voltage signals to said deflection plates, and an electric cylinder lens surrounding said axis within said envelope for providing corrective deflection of said electron beam to compensate for defocussing of said electron beam resulting from the electrostatic deflection of said beam by said deflection plates, said electric cylinder lens comprising two electrodes positioned substantially orthogonal to said axis, each electrode having a slot-like aperture centered relative to the axis, the longer sides of said apertures being substantially orthogonal to the direction of deflection of said electron beam, a flat grid-like electrode positioned between said electrodes substantially orthogonal to the axis, and means including two parallel rectifier elements and a grounded potentiometer, the anodes of said rectifier elements having a common connection in series with said potentiometer, the cathode of each rectifier element being connected to each deflection plate for deriving from said deflection signals a negative voltage having a value as determined by departures of the amplitude of said deflection signals from the average value thereof, and the tap of said potentiometer being connected to said grid-shaped electrode thereby to provide dynamic correction of defocussing of said beam by said deflection signals. 